This is one of the reasons I think pep weapons aren't as popular as just modding something. Only thing I can think of is don't put the two haves together and do the insides. Then put them together using hotglue or something
What I did for my second AR build is I filled it with expanding foam and used the hot glue method on some of hte smaller parts. The only problem I had was I didn't reinforce it so I got some slight warpage on a few small areas that I'm working to fix.
the absolute best and cheapest way to strenghten a weapon is to resin the outside and fill the inside with expandable foam- here this will help-http://405th.com/forums/index.php?showtopic=14344&st=0&p=221737&fromsearch=1&#entry221737
I really wish that people would read things before posting. This is the 3rd or 4th thread with this exact question that has popped up in the past week.